In modern medical applications the use of transcutaneous measurement of blood parameters is becoming of increasing importance. Typically, the outermost layer of a small area of skin on a patient is removed, e.g., by repeated application of a strip of adhesive tape. A measuring device may then be sealed onto the stripped area of the skin so that the environment in the device can equilibrate with the environment of the body fluids at the surface of the skin. Measurements may then be made to determine parameters of interest such as blood pH or CO.sub.2 concentration in the blood. One such device is described in co-pending U.S. patent application Ser. No. 547,670, titled "Medical-Analytical Gas Detector" by Paul E. Stoft et al., and assigned to the present assignee.
In some transcutaneous measuring devices known in the prior art body fluids are drawn into a chamber and brought into contact with a pH sensing glass electrode. With respect to transcutaneous measurements, pH electrodes of this kind are disadvantageous particularly in that problems with reference junction potentials may cause unstable response with time.
IN ACCORDANCE WITH THE ILLUSTRATED PREFERRED EMBODIMENTS, THE PRESENT INVENTION PROVIDES A TRANSCUTANEOUS PH measuring device in which a chamber containing a known volume of fluid is brought into equilibrium with body fluids through a membrane on a portion of skin whose surface layers have been stripped away. The volume of fluid contains a pH-sensitive dye whose optical transmission characteristics at different frequencies depends on the concentration of hydrogen ions in the fluid. The device includes a mechanism to draw the fluid into a chamber positioned between an optical source and an optical detector. When the fluid is in the chamber, optical measurements of its transmission characteristics at different frequencies are obtained. These measurements are then analyzed to provide an indication of the pH of the blood. During certain portions of each operating cycle, the fluid is driven out of the optical chamber and back into the vicinity of the skin by an optical plug of known transmission characteristics. While the fluid again comes into equilibrium with the blood, the optics of the system can be normalized by measurements through the optical plug.